This is an open-label (identity of assigned study drug will be known) study of abiraterone acetate plus prednisone in male patients with metastatic castration-resistant prostate cancer. This study will consist of screening, treatment, and follow-up periods, and will have 2 study groups. Patients in Group A and B will receive daily abiraterone acetate (1000 mg) plus prednisone (5 mg) twice daily beginning on Cycle 1 Day 1 until disease progression. Patients in Group A will take dextromethorphan hydrobromide 30 mg administered orally once daily on Day -8 and Day 8 of Cycle 1. Patients in Group B will take theophylline 100 mg administered orally once daily on Day -8 and Day 8 of Cycle 1. Serial pharmacokinetic samples will be collected and safety will be monitored throughout the study.